A) Field of the Invention
The present invention relates to a transmission line using oxide superconductor which has a low loss and can accommodate a large current flow therethrough.
B) Description of the Related Art
As a high frequency transmission line, a coaxial transmission line is known which has a grounded external conductor surrounding a central conductor. An electric field is generated from the central conductor toward the grounded external conductor. A magnetic field is generated perpendicular to the direction of the electric field. Current flows along an extension direction of the central conductor and grounded external conductor (along a direction perpendicular to the cross section). Known as conductive material are good electrical conductors such as Cu, Ag and Au, and super conductors. A space between the central conductor and grounded external conductor is filled with air or solid state dielectric (hereinafter simply called dielectric). If dielectric is used, the transmission line can be made more compact than using air. The central conductor may have a hollow structure.
FIGS. 4A to 4C are perspective views schematically showing examples of the structure of a transmission line according to prior art.
In FIG. 4A, a cylindrical central conductor 101 and a grounded tubular external conductor 102 are electrically separated by a dielectric block 104. Material having a small high frequency loss is selected as the dielectric. If material having a high dielectric constant is used, the transmission line can be made compact. The grounded external conductor 102 and central conductor 101 are made of normal conductor such as Cu, Ag and Au. Since current in the central conductor 101 flows in the surface layer, the central conductor 101 may have a tubular hollow structure. In this case, the thickness is set to twice a skin depth or thicker. If the central conductor 101 has the hollow structure, dielectric 103 may be filled in the hollow space.
If the conductor is made of superconductor, a superconductor line has a d.c. resistance of 0 and a very small resistance even at high frequencies. It is therefore possible to form a low loss, large current transmission line. Oxide superconductor enters a superconductive state at a relatively high temperature and is convenient for handling.
Oxide superconductor has the electric characteristics very sensitive to the structure of crystal grain boundaries, as different from metal conductor or the like. Many oxide superconductors have a rectangular solid crystal structure. If there are several degrees between crystal axis directions of adjacent rectangular solids, a crystal grain boundary is formed therebetween.
In the structure shown in FIG. 4A, if the dielectric block 103 is made of single crystal and the grounded external conductor 102 is tried to be formed by epitaxially growing oxide superconductor on the arc outer surface of the dielectric block 103, it is very difficult to epitaxially grow oxide superconductor.
FIG. 4B shows another configuration of a transmission line. On the outer surface of a rectangular prism dielectric block 104 preferably made of single crystal, a grounded external conductor 102 of oxide superconductor is formed. An inner hole having a circular cross section is formed through the dielectric block 104, and a central conductor 101 is accommodated in the inner hole. The central conductor 101 may have a hollow structure, and dielectric 103 may be accommodated in the hollow space. A hollow structure without filling the dielectric may also be adopted.
FIG. 4C shows another configuration of a transmission line. A dielectric block 104 preferably made of single crystal has a rectangular prism shape and a rectangular prism inner hole. On the outer surface of the rectangular prism, a grounded external conductor 102 is formed, and on the inner wall of the rectangular prism inner hole, a central conductor 101 is formed. The central conductor 101 has a hollow structure, and dielectric 103 may be accommodated in the hollow space. The grounded external conductor 102 and central conductor 101 are made of oxide superconductor.
The grounded external conductor 102 shown in FIG. 4B and the central conductor 101 and grounded external conductor 102 shown in FIG. 4C are formed on flat surfaces of the single crystal dielectric blocks 104. However, as an oxide superconductor layer is epitaxially grown, the oxide superconductors on adjacent surfaces contact each other at the edge portion of the rectangular prism. If crystal orientations are different, generation of a crystal grain boundary is inevitable. This crystal grain boundary increases a loss and large current is difficult to be flowed. Although an epitaxial layer or a layer near single crystal can be formed on a flat underlay, it is inevitable that crystal grain boundaries are formed at four edge portions.